The present invention is directed to a planar microvacuum sensor based on the principle of thermal conductivity.
Conventional thermal conductivity vacuum gauges are based on the measurement of the pressure-dependent heat elimination of a thinned gas with the assistance of an electrically heated wire erected in the vacuum and have been known as prior art for a long time. Thermal-electrical microvacuum sensors having thin-film heating elements that are manufactured with microtechnological manufacturing methods are also known (for example, A. W. Herwaarden et al., "Small-Size Vacuum Sensors Based on Silicon Thermopiles", in Sensors and Actuators A, 25-27, 1991, 565-569). In these vacuum sensors, a self-supporting membrane or a freely suspended beam of single-crystal silicon is erected on which the thermal-electrical materials and heating films are produced by implantation of the silicon or, respectively, by thin-film deposition. The basic principle of the radiant thermopile is thereby utilized, whereby the interconnects (thermo-legs) produced in thin-film technology for the two different thermal-electrical materials are connected in series such that contacts arise in alternation in the central part of the membrane or on the beam (what are referred to as "hot" contacts) and on the basic silicon member (what are referred to as "cold" contacts). By introducing a heat flow at the "hot" contacts with the heating layer, a temperature increase of these "hot" contacts arises compared to the "cold" contacts on the basic silicon member serving as heat sink. The temperature difference between the "hot" and the "cold" contacts is dependent on the thermal conduction of the gas that surrounds the sensor chip. This thermal conduction decreases with reduced gas pressure and the thermal-electrical signal voltage increases.
A microvacuum sensor wherein the heating wire is provided as a meander-shaped, thin layer of platinum on a thin SiO.sub.2 membrane that is suspended free-floating at four webs of the same SiO.sub.2 is also known (Ping Kuo Wang, Jin-Shown Shie, "Micro-Pirani Vacuum Gague" Rev Sci Instr 65 (2), 1994, 492) The membrane, which is located on a single-crystal silicon wafer, is prepared by anisotropic etching of the silicon. A pyramidal etched trench whose depth is determined by the dimensioning of the membrane and amounts to a few hundred micrometers arises under the membrane as a result of the etching process.
The pressure sensitivity of the described vacuum sensors is fundamentally limited by the following effects:
Toward high pressures, the thermal conduction by the surrounding gas becomes independent of pressure (and, thus, so does the sensor) as soon as the average free path length of the gas becomes smaller than the spacing of the heated surface (for example, the surface of the heating wire, the membrane or beam surface) from the surfaces of the unheated environment. A sensor chip freely positioned in the vacuum or, respectively, a chip having a typical chip thickness of approximately 500 .mu.m secured on a base loses its pressure sensitivity at approximately 10 mbar for said reasons. Thermal conductivity sensors of the described type can therefore not be employed for exact pressure measurements in a low vacuum between 50-1013 mbar.
In the direction toward low pressures, the thermal conduction through the surrounding gas decreases proportionally relative the pressure. In conventional heating wire vacuum gauges, this thermal conduction by the gas already becomes lower than the (pressure-dependent) heat output of the heating wire as a consequence of thermal radiation and as a consequence of thermal conduction via the wire suspension given pressures around 10.sup.-3 mbar. These vacuum gauges are therefore pressure-insensitive below 10.sup.-3 mbar.